An optical rotary encoder generally has a light source; a rotary slit plate which is attached to a motor and is provided with a rotation angle detection tracks formed by optical slits for converting light emitted from the light source into pulsed light; and rotation angle detection light receivers responsible for the photoelectric conversion of the pulsed light thereby formed. The rotation angle detection light receivers have light receiving patterns with a pitch corresponding to the pitch of the optical slit. The rotation angle detection light receivers convert light emitted from the light source and modulated by the optical slits into electric signals, and observe the electric signals thereby converted to detect the rotation angle of the motor.
This type of optical rotary encoder is intended to monitor variations of the light amount of the light source without increasing the outer dimensions of the rotary slit plate and without sacrificing the output of a rotation angle detection signal. As an example, patent publication 1 titled “Absolute Encoder” discloses in its claims an optical absolute encoder including: a rotary slit plate with rotational position detection tracks (rotation angle detection tracks) made of optical slits; light emitting elements (light sources) for emitting light; a fixed slit plate with rotational position detection slits corresponding to the rotational position detection tracks and with light amount monitoring slits for monitoring the light amounts from the light emitting elements; light receiving elements for rotational position detection that are arranged in corresponding relationship with the rotational position detection slits; and light receiving elements for light amount monitoring arranged in corresponding relationship with the light amount monitoring slits. In this optical absolute encoder, the light amount monitoring slits of the fixed slit plate are arranged in a circumferential direction in corresponding relationship with the rotational position detection tracks of the rotary slit plate, and some of the rotational position detection tracks are also operative to function as light amount monitoring tracks. It is also disclosed that the width (angular width) of the light amount monitoring slits in the circumferential direction is an integral multiple of the pitch (angular interval) of the optical slits constituting the rotational position detection tracks that also serve as the light amount monitoring tracks.
Patent Publication 1: Japanese Patent Application Laid-Open No. 6-294666